The present invention relates to a method of maintaining metal or the like in a melted state in a container by the aid of electric resistance heating. Furthermore, the invention relates to a heating element for use with said method.
It is previously known to supply energy to a metal melt by using electric resistance elements which may, in principle, be arranged in two different manners, i.e. either on top of the melt surface or in a tube or another sleeve-shaped body that is partly immersed in the metal melt. Conventional materials in such electric resistance elements are alloys of chrome-nickel and iron-aluminium, as well as e.g. silicon carbide, graphite, and molybdenium-silicon.
With the method comprising electric resistance elements provided on top of the metal melt surface, energy from said resistance elements is transferred to the metal melt by heat radiation onto the melt surface. This means that the container per se for the metal melt must be dimensioned with equal consideration to heat energy transfer and the space required by the actual production process. This will, in turn, result in a metal bath with a content of molten metal considerably larger than necessary for the production process per se. Due to this, much capital will be locked-up, e.g. as in the case of top heated galvanizing furnaces. Additional disadvantages of this method are that the resistance elements are not very resistant to metal spatter from the metal bath, and that any protection of the resistance elements against such metal spatter will result in reduced heat transfer from said elements to the metal melt.
In another known method the metal element is supplied with energy by heating elements/resistance elements, preferably shaped as rods, and provided in a tube with a bottom or in another sleeve-shaped body that is partly immersed in the metal melt, and where there is no electroconductive contact between the resistance element(s) and said sleeve. With this method heat is transferred by radiation from the resistance element to said sleeve from which heat conduction occurs in the metal melt.
Heating elements of the above kind are disclosed, inter alia in U.K. No. 1 027 163 and U.S. Pat. No. 4 132 886. Said sleeves may be manufactured from different kinds of material. When the material comprises metal alloys the sleeve temperature will be limited to a relatively low level, causing a reduction of the amount of energy transfer that could otherwise be utilized from said resistance elements. Another disadvantage of metal alloys is that they are not very resistant to metal melts, e.g. from zinc and aluminium. Tubes or sleeves made from a material based on, e.g. graphite, silicon carbide, silicon nitride, or aluminium nitride resist higher temperatures, and may also be resistant to molten metals. In practice, however, it proved difficult to achieve a satisfactory tight tube or sleeve in said materials. This will, inter alia, result in the fact that the outer tube or sleeve surface facing the metal melt is subjected to oxygen resulting in an oxidation of said outer surface and/or the molten metal adjacent said outer surface. The oxide layer, so, formed has a heat insulating effect and will, thus, reduce the amount of transferred energy. Another disadvantage of such tubes or sleeves is that the heat exchange constancy may be poor.